1. Field of the Invention
The invention relates to methods and tools used in knee arthroplasty. More particularly, the invention relates to methods and tools used in revision surgery where an artificial tibial component is removed and replaced. The invention also relates to improved tibial components.
2. Brief Description of the Prior Art
Total knee arthroplasty involves the replacement of portions of the patellar, femur and tibia with artificial components. In particular, a proximal portion of the tibia and a distal portion of the femur are cut away (resected) and replaced with artificial components. As used herein, when referring to bones or other body parts, the term "proximal" means closest to the heart and the term "distal" means more distant from the heart. When referring to tools and instruments, the term "proximal" means closest to the practitioner and the term "distal" means more distant from the practitioner.
There are several types of knee prostheses known in the art. One type is sometimes referred to as a "resurfacing type". In these prostheses, the articular surface of the distal femur and proximal tibia are "resurfaced" with respective metal and plastic condylar-type articular bearing components. These knee prostheses provide adequate rotational and translational freedom and require minimal bone resection to accommodate the components within the boundaries of the available joint space.
The femoral component is a metallic alloy construction (cobalt-chrome alloy or 6A14V titanium alloy) and provides medial and lateral condylar bearing surfaces of multi-radius design of similar shape and geometry as the natural distal femur or femoral-side of the knee joint.
The tibial component usually includes a distal metal base component and a proximal interlocking plastic, e.g. UHMWPE (ultra high molecular weight polyethylene), component or insert. The plastic tibial plateau bearing surfaces are of concave multi-radius geometry to more or less match the articular geometry of the mating femoral condyles. Both the femoral and tibial components are usually provided with intermedullary (IM) stem options.
After preparing the distal surface of the femur and the proximal surface of the tibia, an opening is made into the medullary canal of the femur, and an opening is made into the medullary canal of tibia. The interior surface and the IM stem of the femoral component are usually covered with a polymeric cement and the IM stem is inserted into the medullary canal of the femur until the interior surface of the femoral component meets the distal surface of the femur. The tibial component is similarly usually cemented to the proximal surface and medullary canal of the tibia.
Occasionally, the components are press fit without the use of cement. The use of cement has advantages and disadvantages. Press fit components rely on bone quality to obtain good fixation. Sometimes it is impossible to obtain good fixation with a press fit component and sometimes a press fit component will fail early because of failure of successful biological ingrowth. Cement assures good fixation but puts strain along the component stem. In addition, as described below, cement complicates the removal of a failed component.
Often, due to normal wear over time, the prosthetic knee must be replaced via a procedure known as revision surgery. When the primary cemented prosthetic is removed, the proximal surface of the tibia and the distal surface of the femur typically exhibit cavernous defects. Absent the use of bone graft, the proximal surface of the tibia and the distal surface of the femur must be carefully resected to remove cavernous defects before a replacement knee can be installed.
In addition, the intramedullary (IM) canals must be broached or reamed to remove any remaining cement or cavernous defects existing in the canals before a replacement knee can be installed.
According to the state of the art, after the primary prosthetic is removed, the proximal tibia is resected with a lateral template. The medullary canal is reamed and the reamer is tapped in place with a mallet. A proximal resection guide is attached to the reamer and proximal resection is completed via slots in the guide. Preparation of the distal femur is described in the above referenced related application.
The defects in the tibia are measured and the cutting guide is moved down 6 to 10 mm. A flat cut from anterior to posterior is made. A tibial template is attached to the reamer and reference marks are typically in pen. A flat cut and sagittal cut are made relative to the reference marks. Another template is attached to the reamer and anterior and posterior holes are drilled for securing a wedge resection guide. A wedge cut is then made. The template is replaced and aligned with the marks. A revision mask punch guide is attached to the template and a revision box chisel is used to prepare for a stem.
Those skilled in the art will appreciate that revision surgery is difficult because (1) the type and location of cavernous defects make it difficult to match the exterior surfaces of the tibia and femur to the interior surfaces of the prosthetic, (2) the femur and tibia must be resected with reference to the IM canal, and (3) the use of multiple templates and guides during the course of the procedure makes it very difficult to keep all the cuts in proper alignment relative to the IM canal.
In particular, with respect to the tibia, resection of the proximal tibia results in the creation of a tibial plateau in which the IM canal is no longer centrally located. If a normal tibial component is installed, portions of the tibial component will overhang the resected tibial plateau.
In order to compensate for this problem, it is known in the art to provide tibial components with offset IM stems. However, the relative location of the IM canal relative to the perimeter of the tibial plateau may be offset in any direction, anterior, posterior, medial, or lateral, depending on the individual bone. It is impossible or at least impractical to provide an offset stem tibial component for every possible variation in the relative location of the IM canal.
Moreover, it is difficult to estimate the offset of the IM canal in order to choose an appropriate offset tibial component.
According to the state of the art, offset tibial components are selected by trial and error, a tedious procedure which prolongs surgery. For example, as shown in FIG. 1, a relatively symmetrical tibial plateau 10 exhibits the IM canal 12 in a central location. After resection of the tibial plateau, the location of the IM canal may be located off center as shown in FIGS. 2 and 3 where canals 12' and 12" are seen to be located off center relative to the plateaus 10' and 10" respectively.